1. Field of the Invention
The present invention relates to a method for fabricating a micro-electro-mechanical structure, and more particularly to a suspension microstructure and its fabrication method, which can effectively avoid improper erosion, and further simplify the mask and reduce the package cost.
2. Description of the Prior Art
The existing semiconductor micro-electro-mechanical systems comprises various semiconductor microstructures, such as the unmovable probe, channel, chamber structure, or the movable spring, linkage rod, gear (rigid body movement or flexible deformation), etc.
Integrating the above different structures with the related semiconductor circuit can form various semiconductor applications. Thus, how to utilize the fabricating method to improve the various functions of the microstructure is the key index of the semiconductor electromechanical system in the future and is also a rigorous challenge of further developing the chip in the future.
The existing method for fabricating the micro-electro-mechanical sensor and actuator system is often required to fabricate a suspension structure on a silicon substrate. The above process must adopt the advanced semiconductor technology, such as: wet etching, dry etching and sacrificial layer removing technology, etc.
The wet etching is a fast and effective etching without etching the etchants of other materials. Thus, the wet etching has a quite selectivity according to different materials. However, excluding that the crystallization direction is likely to affect the etching rate, because the chemical reaction doesn't vary in some specific directions, the wet etching is an isotropic etching in essence. The isotropic etching implies that the wet etching can have both the longitudinal etching effect and the transverse etching effect. The transverse etching will cause the undercut problems.
On the contrary, in the dry etching (plasma etching), the plasma is partially-dissociated gas. The greatest advantage of the dry etching is anisotropic etching. However, the selectivity of the dry etching is lower than that of the wet etching (the etching process of the dry etching is generally a physical interaction, so the ion collision can not only remove the etched film, but synchronously remove the photoresist).
The conventional fabrication method of the micro-electro-mechanical structures must need fabricating a layer of precision mask for restricting the etching, but the mask technology is different from the technology of the deposition of the insulation layer. The precision mask fabrication must apply the complicated technology and equipment (such as exposure machine, optical draft machine) to map the patterns of the masks on the insulation layers of the wafers precisely. Therefore, the recent micro-electro-mechanical structure manufacturers mostly apply another process (or different manufacturer) to fabricate the masks, so that the mask becomes more and more elaborate, thus increasing the manufacturing cost and limiting the dimension and precision of the microstructures.
A first conventional method disclosed in U.S. Pat. No. 6,458,615 B1 is to form at least one insulation layer including inner micro-electro-mechanical structures on an upper surface of a silicon substrate, and then conduct a layer-by-layer etching operation from the upper surface until the lateral edge of the micro-electro-mechanical structure, and finally, conduct an isotropic dry etching to the silicon substrate to realize the suspension of the micro-electro-mechanical structures.
The first conventional method can be used to fabricate a suspension micro-electro-mechanical structure, but it has the following disadvantages:
1. It adopts anisotropic dry chemical etching and uses chemical reaction to remove the isolation layer, however, after the side edge of the micro-electro-mechanical structure is etched, the silicon substrate still needs to be massively etched by isotropic chemical etching, and this technique will produce serious undercut problems;
2. In the process of this conventional technology, the micro-electro-mechanical structure is exposed in the process at first, after a long time of multi-layer processing, the exposed micro-electro-mechanical structure is likely to be contaminated and damaged, causing an excessively low yield rate;
3. The exposure of the metal layer during the process of etching is likely to cause contamination of the chamber of the etching machine by metal and cause unstable etching rate.
With the rapid development of the above technology, in order to improve many problems, a second conventional method disclosed in U.S. Pat. No. 6,712,983 B2 taught the use of a reactive ion etch (hereinafter referred to as RIE) technology. This technology is capable of greatly reducing occurrence of undercut, but since it also conducts a layer-by-layer etching from up down, and the last etching operation of the silicon substrate must apply the transverse etching technology, this improved conventional technology is still complicated and has the undercut problem. It is important that, some problems still have not been improved, such as: the exposure of the micro-electro-mechanical structure and affecting yield rate, etc.
The problem that cannot be solved by the existing technologies is that, a layer of precision mask is still needed for precisely finishing the etching operation, but the micro-electro-mechanical structure has become more and more elaborate, so the fabrication of the mask becomes more and more difficult, thus not only increasing the manufacturing cost, but also causing troubles in transmission, error, cost and particle contamination.
In addition, the shorter the distance between the mask patterns is, the smaller the etching hole will be. The reactant and the ion with energy in the etching system cannot reach and contact the predetermined bottom, or the reaction products cannot be fully discharged out of the hole, thus leading to the reduction of the etching rate. The smaller the etching hole is, the more serious the above phenomena will be, which is the so-called micro loading effect.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.